The present invention relates to electrostatic development toner, and more particularly to one-component nonmagnetic toner for use in electrophotography, electrostatic printing, etc.
In electrophotography, electrostatic printing, etc., an electrostatic latent image is formed on the surface of a photosensitive member by various methods, and is developed by toner to provide a toner image which is then transferred onto a plain paper, etc. and fixed thereto.
The development of an electrostatic latent image is usually carried out by a magnetic brush method, and in this magnetic brush method, a two-component developer composed of a carrier such as iron powder and ferrite powder and toner consisting mainly of a resin and a colorant is widely used. In the case of a two-component developer, however, a mixing ratio of the toner to the carrier (toner concentration) should be strictly controlled to obtain a stable image, necessitating a control means for toner concentration. Further, the carrier surface is contaminated by the toner after a long period of use, resulting in the decrease in the triboelectric charging ability of the carrier.
To eliminate these problems, a one-component development method using magnetic toner consisting essentially of a resin and magnetic powder has been developed and widely used. In the one-component development method, insulating magnetic toner is usually used to achieve good image transfer (see, for instance, U.S. Pat. No. 4,121,931). With respect to development with magnetic toner, various methods have been proposed. Among them, a method of triboelectrically charging the toner by friction between toner particles themselves or between toner particles and a sleeve, etc. is widely used.
The development method utilizing the triboelectric charging of the toner, however, suffers from the disadvantage that the toner tends to agglomerate, and that the image quality varies with environmental conditions. Further, since the magnetic toner contains 20-70 wt % or so of magnetic powder such as magnetite, it is inferior to the two-component developer in fixability.
Because of these problems, much attention has been paid to a nonmagnetic one-component toner containing no magnetic powder. Known development methods using such nonmagnetic one-component developer are an impression development method, a touchdown development method and a jumping development method. However, the impression development method and the touchdown development method are likely to cause fogging because a toner-bearing member and a photosensitive member are in contact with each other. And in the jumping development method, it is necessary to narrow the gap between a toner-bearing member and a photosensitive member and to supply a thin layer of the toner to this gap to increase a development efficiency. However, it is usually difficult to form a uniform thin layer of the toner.
To eliminate these problems, investigations have used as a toner-bearing member a sleeve equipped with a plurality of floating electrodes on the surface. As shown in FIG. 2, a sleeve 1 is constituted by a cylindrical substrate 2 made of nonmagnetic and conductive materials such as aluminum, stainless steel, etc., a dielectric layer 3 formed on the cylindrical substrate 2, and fine electrodes 4 constituted by a large number of conductive particles dispersed in a mutually insulated state on the surface of the dielectric layer 3. The sleeve of such structure is disclosed, for instance, in Japanese Patent Laid-Open No. 60-225179. In an apparatus shown in FIG. 1, sleeve 1 is rotated and a bias voltage source 7 is connected between the sleeve 1 and a doctor blade 6. In this apparatus, toner 8 contained in a hopper 5 is fed onto the rotating sleeve 1 while the doctor blade 6 regulates a thickness and triboelectric charge of the toner 8, whereby an electrostatic latent image (not shown) formed on the surface of the photosensitive drum 10 is developed. According to this development method, sufficient edge effects can be stably obtained, providing good pictorial and linear images.
Despite the fact that good images can be obtained by a development method using nonmagnetic one-component developer and a sleeve provided with a large number of electrodes, it is disadvatageous for the following reasons.
Since the usual nonmagnetic one-component toner is composed mainly of a binder resin and a colorant, the toner tends to adhere to the sleeve surface after a long period of use, thereby forming a thin insulating layer on the surface of the sleeve. As a result, even if a bias voltage is applied to the sleeve, the electric field between the fine electrodes and the photoconductive drum is weakened, resulting in decrease in a image density.